In recent years, with the critical situation of the worsening of the global environment with global warming and the like, the construction of systems with sustainable use of materials has been accelerated from the viewpoint of using effectively the limited carbon resources and conserving limited energy resources. In the case of polymer products, after use the products are reused as they are (examples of this case include the conversion of PET bottles to fibrous material), or recycled or discarded. For recycling processes, material recycling processes, chemical recycling processes, thermal recycling processes, and the like are used. However, material recycling processes involve a deterioration in quality, such as a drop in molecular weight, chemical recycling processes consume much energy, and thermal recycling process generate a large amount of carbon dioxide gas. Thus, each of these processes involve problems.
From the viewpoint of effective use of carbon resources, the ideal is that products are finally restored to the raw materials thereof by chemical recycling processes. Regarding chemical recycling processes, the recovery of monomers by depolymerization reactions, and recovery of monomers as raw materials by chemically decomposition reactions are known. However, in the case of performing depolymerization based on chemical decomposition or thermal decomposition, both terminals of a generated low molecular weight compound are irregular. It is therefore impossible to re-polymerize such compounds as they are, and it is necessary to conduct isomerization reactions, purification thereof or some other operations. For example, when polyester is hydrolyzed at high temperature and high pressure in the presence of NaOH, carboxylic acid yielded is present as a Na salt thereof. Thus, this acidity must be neutralized. Accordingly, the processes consume much energy, and cause the discharge of inorganic salts (such as NaCl), and other problems. Thus, the processes impose large loads upon environment, and are generally unprofitable.
Apart from the viewpoint of reuse, attention has been paid to the so-called biodegradable polymers, which are degraded by bacteria and the like in the ground, as polymers imposing only small loads on environment. Various such biodegradable polymers have been suggested. For example, for biodegradable polymers biodegradable polyesters are known. Typical examples of biodegradable polyesters produced by chemical synthesis include polycaprolactone (PCL), polylactic acid (PLA), polyhydroxybutyric acid (PHB), and aliphatic polyesters made from diol and succinic acid, such as polyethylene succinate, polybutylene succinate (PBS) and polybutylene succinate/adipate copolymer (PBS/A). Among these, as well as polylactic acid and polycaprolactone, polybutylene succinate (PBS) has been investigated in an attempt to make it practicable as a typical chemically-synthesized biodegradable plastic, since PBS can be obtained from 1,4-butanediol and succinic acid by a petrochemical industrial process.
Among the above-mentioned examples, poly(L-lactic acid) (PLA) is a polymer yielded by polymerizing lactic acid, which is obtained by fermenting corn starch, or the like, which is a renewable resource. It can be said that this polymer is a low environment load polymer, which does not cause a direct increase in the total amount of carbon dioxide gas even if the polymer is finally biodegraded or burned up. Lactic acid or a dimer thereof, which is the raw material of the polymer, has already been produced with a high efficiency by research and development over many years.
Polylactic acid is a biodegradable plastic which has strength equivalent to that of polyethylene or polystyrene, has a higher transparency than other biodegradable plastics, and is superior in weather resistance, heat resistance, workability and the like. Polylactic acid has already been put to practical use such as in covering materials for agriculture, fibers, earth-retaining netting, weed-preventing bags and the like. Accordingly, polylactic acid is a biodegradable plastic that has currently been developed furthest towards practical use.
Polylactic acid is degraded to water and carbon dioxide with several years in the ground, or in a short period in compost. Therefore, covering materials and the like for agriculture, which are used outdoors, can be left as they are after they have been used. However, polylactic acid has a considerably lower biodegradability than polycaprolactone or polyhydroxybutyric acid. Thus, there is the fear that a large amount of polylactic acid left outdoors may cause a new environmental problem.
There is a chemical recycling process in which polylactic acid is thermally decomposed to regenerate the monomer thereof. This process requires a high temperature of 270° C. or higher, and consumes much energy. Thus, it is difficult to say that this process is superior in recycling. Although polylactic acid can be obtained from a renewable resource, it is impossible to ignore energy applied to the production thereof, including the cultivation of plants for producing starch as the raw material, the harvesting thereof and production by fermentation and the like. Moreover, the raw materials of a biodegradable polymer such as polylactic acid are not recovered, although the biodegradable polymer imposes only a small load on the environment. Accordingly, the process does not fall under the category of complete-cycle type of reuse, wherein carbon resources are effectively used. Thus, it is hard to say that the process is an ideal polymer-degrading process.
Accordingly, a polymer producing/degrading process can be constructed, which is one of both low energy consumption and is also a complete-cycle type, if the following can be attained: a polymer can be degraded into a low molecular weight compound without using high energy, such as petroleum energy, like biodegradable polymers; the low molecular weight compound can be effectively utilized; and, if desired, the original polymer can be obtained from the low molecular weight compound without similarly consuming any high energy.
Incidentally, Japanese Patent Application National Publication (Laid-Open) No. 2001-512504 describes, as a process for degrading a biodegradable polymer a process of degrading various biodegradable polymers, such as aliphatic or partially-aromatic polyesters, thermoplastic aliphatic or partially-aromatic polyester urethanes which may contain a urethane group, aliphatic-aromatic polyester carbonates and/or aliphatic or partially-aromatic polyester amides, by use of an enzyme such as lipase in an aqueous enzyme solution in which a buffer may be contained. However, the degrading technique described in this publication is a technique for degrading a biodegradable polymer rapidly in an aqueous solution by using an enzyme, and, for example, for the use in a method of degrading and removing a biodegradable polymer from a complex of the biodegradable polymer and a different useful material (such as a metal), thereby recovering the useful material easily. The main aim thereof is not to reuse products generated after the degradation. It is also difficult to use the product decomposed by this process for re-polymerization or the like.
The above-mentioned publication states that fine particles of polylactic acid, which is a biodegradable polymer, is degraded in the presence of a specific lipase in a potassium phosphate buffer solution. However, it is a well-known fact that polylactic acid is not degraded under ordinary conditions even if microorganisms are present therein and polylactic acid is not degraded by microorganisms without, for example, conditions of high-temperature and high-humidity. It is thought that this is because polylactic acid is first hydrolyzed at high temperature and high humidity, so that a fall in the molecular weight occurs, and it is not until the stage at which the hydrolysis advances that polylactic acid starts to be degraded by the participation of microorganisms (a two-stage reaction) (J. Lunt, Polymer Degradation and Stability 59, 145-152 (1998)). It appears that the lipase degradation of polylactic acid in a potassium phosphate buffer solution, described in the above-mentioned publication, is based on the following: polylactic acid is first hydrolyzed so that a fall in the molecular weight thereof occurs since polylactic acid is made into very fine powder, and lipase acts on the low molecular weight polylactic acid. (The only enzyme capable of degrading high molecular weight polylactic acid which is known is protease K.)
The rapid degradation of polylactic acid in compost by microorganisms is caused by the occurrence of a two-stage reaction as described above, since conditions of high temperature and high humidity are maintained in the compost.
Accordingly, it is considered that no enzyme other than protease K can be caused to act directly on polylactic acid so as to degrade the acid. The mainstream of practical research is about degradation of polylactic acid in compost. Currently, research into causing enzymes to directly act on polylactic acid to degrade the acid is not being carried out, excepting research in which the degradation by use of the above-mentioned protease K is used for simplified evaluation of biodegradability. Additionally, research based on a concept that polylactic acid is depolymerized with an enzyme to recover a re-polymerizable oligomer has not yet been undertaken. Accordingly, there has not yet been suggested any chemical recycling process of depolymerizing polylactic acid with an enzyme to yield a re-polymerizable oligomer.
Correspondingly, the inventor of the present invention previously suggested a process for depolymerizing a polymer with an enzyme, wherein a depolymerization product generated by depolymerizing the polymer has re-polymerizability. Japanese Patent Application Laid-Open (JP-A) No. 2002-17385 describes a depolymerizing process of a caprolactone polymer with a hydrolase. By this depolymerization, dicaprolactone is produced in a high yield, and the produced dicaprolactone can be re-polymerized with an enzyme. Japanese Patent Application No. 2001-131768 relates to a process of depolymerizing polyalkylene alkanoate or poly(3-hydroxyalkanoate) in the presence of a hydrolase to produce re-polymerizable oligomer(s) including, as main component(s), cyclic oligomer(s). Furthermore, Japanese Patent Application No. 2002-193114 relates to a process of depolymerizing polyester or polycarbonate in the presence of a hydrolase in a supercritical fluid. A cyclic oligomer can be re-polymerized with an enzyme in a supercritical fluid.
These polymers are conventionally well known as biodegradable polymers which are easily degraded by microorganisms, and the behavior thereof to microorganisms is greatly different from that of polylactic acid.